Studies have been made on application of a ZnO-based semiconductor device using ZnO, which is a type of oxide, to an ultraviolet LED used as a light source for illumination, backlight or the like, a high-speed electronic device, a surface acoustic wave device, and so forth. ZnO has drawn attention to its versatility, large light emission potential and the like. However, no significant development has been made on ZnO as a semiconductor device material. The largest obstacle is that p-type ZnO cannot be obtained because of difficulty in acceptor doping. Nevertheless, as demonstrated by Non-patent Documents 1 and 2, technological progress of recent years has made it possible to produce p-type ZnO, and has proven that light is emitted from the p-type ZnO. Accordingly, active research on ZnO is underway.
A proposal has been made on use of nitrogen as an acceptor for obtaining p-type ZnO. As disclosed in Non-patent Document 3, when ZnO is doped with nitrogen as an acceptor, the temperature of the substrate needs to be lowered because the efficiency of nitrogen doping heavily depends on a growth temperature. However, the lowering of the substrate temperature degrades crystallinity, and forms a carrier compensation center that compensates the acceptor. As a result, nitrogen is not activated. This makes the formation of a p-type ZnO semiconductor layer, itself, extremely difficult.
With this taken into consideration, Non-patent Document has disclosed a method of forming a p-type ZnO-based semiconductor layer with a high carrier density by using a −C plane as a main surface for growth under such temperature control that a growth temperature is alternately changed between 400° C. and 1000° C., the method thereby taking advantage of the temperature dependency of the efficiency of nitrogen doping. However, this method involves the following problems. The continuous process of heating and cooling results in the alternate repetition of thermal expansion and contraction of the manufacturing machine. This imposes heavy burden on the manufacturing machine. For this reason, the manufacturing machine requires an extensive configuration, and periodic maintenance service at shorter intervals. Furthermore, the method requires the temperature to be accurately controlled because the doping amount is determined by a part of the process at the lower temperature. However, it is difficult to control the temperature so that the temperature will reach 400° C. and 1000° C. accurately in a short time period, and the reproducibility and stability of the doping thus become inadequate. Further, since the method uses a laser as a heating source, the method is not suitable for heating a large area. In addition, it is difficult to grow multiple semiconductor films, although the growth of multiple semiconductor films is needed to reduce device manufacturing costs.    Non-patent Document 1: A. Tsukazaki et al., Japanese Journal of Applied Physics vol. 44 (2005) L643.    Non-patent Document 2: A. Tsukazaki et al., Nature Material vol. 4 (2005) 42.    Non-patent Document 3: K. Nakahara et al., Journal of Crystal Growth 237-239 (2002) p. 503.    Non-patent Document 4: Toshio Kamiya, “High-performance Materials”, Vol. 24 No. 4, Chapter II “Transparent oxide semiconductor: A new frontier opened by transparent conductive oxides.”